Papers by Author: Mohamad El Mehtedi

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Authors: Mohamad El Mehtedi, Samer El Mohtadi, Stefano Spigarelli
Abstract: A new constitutive relationship based on the combination of the Garofalo and Hensel-Spittel equations has been developed and successfully used to model the plastic flow of a AA6082 aluminum alloy. Two regimes of temperature and strain rate were identified: the constitutive analysis suggested that in the low strain rate/high temperature regime, deformation was controlled by viscous glide of dislocations in atmospheres of Mg solute atoms, while in the high strain rate/ low temperature regime, deformation was controlled by climb.
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Authors: Carlo Bruni, Mohamad El Mehtedi
Abstract: The necessity to simulate extrusion processes requires the knowledge of the material in terms of the constitutive equations. The present investigation deals with the development of a methodology based on the multivariable regression analysis in order to predict the flow behaviour of a ZEK200 Mg alloy in the interval of temperature varying between 150 and 450°C. The proposed models relate the flow curve levels with temperature, strain rate and deformation with the variables selected using the statistical method. It has been found that the proposed methodology allows the modeling of the flow behaviour of the Mg alloy under investigation from the beginning of the plastic deformation down to the softening phase taking into account the peak value. In addition, it is comparable, in terms of the temperature intervals considered in the modeling, with the Sellars and Tegart-based models.
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Authors: Tommaso Pinter, Mohamad El Mehtedi
Abstract: Constitutive equations relate thermo-mechanical parameters, i.e. strain (ε), strain rate (έ) and temperature (T), with flow stress (σ). The most popular constitutive relationship, among those used in the study of hot deformation, is the phenomenological sinh Garofalo equation. In recent years several papers described the hot deformation of aluminum alloys by mean of hot torsion testing. However, sinh constitutive parameters are seldomly available in literature even for the commonest aluminum alloys used today by the extrusion industry. This paper presents the result of the torsion tests providing constitutive equations for AA6005A, AA6063 and AA7020 alloys. The relative extrudabilities of these alloys were estimated by means of FEM simulation carried out by the HyperXtrude® software.
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Authors: Carlo Bruni, Lorenzo Donati, Mohamad El Mehtedi, M. Simoncini
Abstract: The present investigation aims at studying and modelling the flow behaviour of the AZ31 magnesium alloy by means of torsion tests performed in extended ranges of temperature and strain rates. Two types of rheological models were considered. The former is based on the power law equation, whilst the latter is based on the Sellars and Tegart approach. The effectiveness of the two constitutive models in describing the flow behaviour of the AZ31 magnesium alloy under investigation was evaluated. It was observed that both the equations are able to predict the flow behaviour of the material at different temperatures and strain rates. In particular, the former is very effective in predicting the hardening stage of the flow curve, whilst the latter allows to fit the softening stage. The models were used for the finite element analysis of a complex extrusion process and the results, in terms of the load-stroke curves, compared to each other.
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Authors: Galia Harel, Menachem Bamberger, Y. Rami, S. Spigarelli, Mohamad El Mehtedi, Giuseppe Cupitò
Abstract: The market share of wrought Magnesium products such as structural and functional components is recently increasing. Extrusion at elevated temperatures is used to produce reliable plastic deformation, since magnesium alloys have limited ductility at room temperature. In order to produce sound extruded products, high quality billets are required. Understanding the influence of direct chill casting conditions on the production properties such as quality, safety, workability and microstructure have a profound importance. Comprehensive computer simulations were used in order to model the casting so that process parameters can be identified and controlled, resulting in significant benefits. The aim of modeling is to provide temperature profiles for a more accurate solidification analysis, predict the solidification time and the effect of cooling on the solidification. The experimental study included castings of several Magnesium alloys, each with 7 (seven) thermocouples that were submerged into the billet. Verification of the simulations was carried out based on the data collected. Complimentary work was conducted on microstructure analysis in as cast and as-extruded states.
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Authors: P. Ricci, Mohamad El Mehtedi, L. Barone, S. Spigarelli
Abstract: The formability of AZ31 magnesium alloy sheets, with two different thicknesses, has been investigated at room temperature and 250°C by means of Nakazima tests. The different straining conditions have been studied by using sheet blanks with several length to width ratios, and Forming Limit Diagrams were then obtained with and without using lubricant. As expected, an increase in temperature was observed to enhance the formality of the alloy. The formability increases also by increasing the thickness as well as by using Teflon foil as lubricant. The microstructure of the deformed samples was analysed by means of light optical microscope.
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Authors: Carlo Bruni, Mohamad El Mehtedi, Filippo Gabrielli
Abstract: The present investigation deals with the development of a methodology to predict the flow behaviour of the ZM21 magnesium alloy in given intervals of temperature and strain rate by FEM simulation of torsion testing. Equations based on the hyperbolic sine of flow stress and on the multiple linear regression were proposed and implemented into the finite element code. The flow curve shapes obtained by simulation were compared with experimental ones that were not used in the building phase of the equations. It was found that the simulation of torsion tests allows, under given conditions of temperature, strain rate and deformation levels, to obtain flow curve shapes very similar to those obtained by experiments under conditions not included in the building of the models.
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Authors: Archimede Forcellese, Mohamad El Mehtedi, M. Simoncini, S. Spigarelli
Abstract: The formability of AZ31 magnesium alloy sheets has been investigated in the temperature range varying from 200 to 300°C. Forming limit diagrams have been obtained by performing Nakazima-based tests. The different straining conditions have been investigated using sheet blanks with several length to width ratios. The forming limit curves have been related to the microstructural evolution occurring during deformation. The forming limit diagrams have shown a remarkable increase in formability with temperature that could be related to the occurrence of full dynamic recrystallization at 300°C.
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Authors: Carlo Bruni, Alessio D'Orazio, Mohamad El Mehtedi
Abstract: The present investigation aims at studying the effect of the tool geometry and of rotational velocity of the tool, at different welding velocities, on the tensile shear strength of the friction stir welded joints realised with blanks of different thicknesses. The proposed trial and error methodology was based on experiments, numerical simulations and microstructure observations.It was observed that, at the lowest rotational velocity, the slender tool determines tensile shear strength values lower than those obtained with the thick tool in particular at the highest welding velocity investigated. The numerical simulation evidenced a wider stirred zone for the thick tool when compared with that realised with the slender tool at the lowest rotational velocity. Microstructure observations evidenced that the increase in the welding velocity determines reduced stirred zones and an homogenisation of material particularly relevant for the slender tool.
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Authors: S. Spigarelli, Mohamad El Mehtedi, D. Ciccarelli, Menachem Bamberger, Giuseppe Cupitò, Y. Rami
Abstract: The high temperature response in torsion and creep of two extruded Mg-Zn alloys was investigated in the present study. The alloy 0 (Mg-2Zn-1Mn) was found to exhibit a lower strength than the alloy 2 (Mg-0.55Zn-0.79Mn-0.75Al-0.17Ca), even if the activation energy for creep was similar for both materials (170-180 kJ/mol). The difference in flow stress was here preliminarily attributed to the precipitation of fine Al2Ca particles.
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